In a Long Term Evolution (LTE) system, when user equipment (UE) in a radio resource control (RRC) idle state needs to send uplink data, the UE first needs to implement uplink synchronization and establish an RRC connection. This procedure is a random access procedure. When the UE needs to be handed over to another cell, reestablish an RRC connection, re-obtain uplink synchronization, request an uplink resource for sending uplink user data, or the like, a random access procedure may also need to be executed.
The random access procedure executed in the foregoing several cases is generally a contention-based random access procedure. As shown in FIG. 1, a contention-based random access procedure mainly includes a process of sending four messages. These four messages are, respectively, a random access preamble message (msg1 for short) sent by UE to a base station, a random access response message (msg2 for short) sent by the base station to the UE, a scheduled message (msg3 for short) that is used to carry uplink user data and is sent by the UE to the base station, and a contention resolution message (msg4 for short) sent by the base station to the UE. The following details the process of sending these four messages.
When preparing to access a wireless network, the UE obtains 64 available random access preambles from a cell broadcast message that is broadcast by the base station. These 64 random access preambles are divided into two groups based on a size of the msg3. The UE randomly selects a random access preamble from one of the two groups according to an amount of data that is to be sent by the UE, and sends the random access preamble to the base station by adding the random access preamble to the msgl.
After the UE sends the random access preamble by using the msgl, the base station calculates a timing advance (TA), and sends the msg2 to the UE. The msg2 includes a detected preamble index of the random access preamble, the TA, and uplink grant information for transmission of the msg3. After receiving the msg2, the UE sends the msg3 on a time-frequency resource indicated by the UL-grant. The msg3 includes the uplink user data.
The UE may be UE in an idle state or in a connected state. If the UE is in the idle state, in the msg2, the msg2 sent by the base station further includes a temporary cell radio network temporary identifier (Temporary C-RNTI) of the UE; and in the msg3, the uplink user data sent by the UE includes a common control channel-service data unit (CCCH-SDU) and a preset pilot signal. If the UE is in the connected state, in the msg3, the uplink user data sent by the UE includes a C-RNTI and a preset pilot signal.
After detecting the msg3, for the UE in the idle state, the base station sends, to the UE, the CCCH-SDU carried in the msg3. After successfully detecting the msg4, the UE determines that the CCCH-SDU is data previously sent by the UE, and uses, as an actually used C-RNTI, the temporary C-RNTI received in the msg2. For the UE in the connected state, the base station uses a prior C-RNTI of the UE, and uses a physical downlink control channel (PDCCH) to implement contention resolution.
In the foregoing process, if a plurality of UEs select a same random access preamble, and send the same random access preamble to a base station on a same random access resource, after detecting the random access preamble, the base station cannot determine that the random access preamble is sent by a plurality of UEs. In this case, the base station calculates a TA according to a maximum multipath location at which signal energy is detected, and sends the msg2. Afterwards, all the plurality of UEs detect the preamble index carried in the msg2, and send the msg3 according to the TA sent by the base station and the time-frequency resource indicated by the UL-grant. Pilot signals used in the msg3 by the plurality of UEs are also the same. After the base station receives, on the time-frequency resource indicated by the UL-grant, the msg3 sent by the plurality of UEs within a maximum delay extension range, because the pilot signals used by the plurality of UEs are the same, the base station cannot correctly demodulate uplink user data sent by each UE. As a result, random access procedures of the plurality of UEs may all fail.